I found an interesting post concerning this issue:

http://www.gossamer-threads.com/...i?post=236013#236013

I've changed the code in my header template like this:

<%ifnot auth_no_authentication%>
<%if use_cookie%>
<%if Username and Username ne default%>
Currently logged in: <%Name%>
<%endif%>
<%elsif session_id%>Currently logged in: <%Name%>
<%endif%>
<%endif%>


This code can be used both with the sessions and cookies.

Have fun, Oliver